The discovery of the unique catalysis afforded by the acetic acid solution of the joint use of one or more heavy metal oxidation catalysts and a source of bromide ion for the liquid phase oxidation at a temperature from 50.degree. to 275.degree.C. of aliphatic-substituted aromatic compounds with molecular oxygen to aromatic polycarboxylic acid products was first disclosed in U.S. Pat. No. 2,833,816 which issued May 6, 1958. The use of said unique catalysis for such oxidation of xylenes under liquid phase conditions at 50.degree. to 275.degree.C. made feasible for the first time by catalytic liquid phase air oxidation large scale commercial production of the benzene dicarboxylic acids: ortho-, iso- and terephthalic acids. Since 1958 many improved modes of conduct of such oxidations using the unique combination of heavy metal and bromide ion have been disclosed as advancements of that art. Some improvements were directed to yield improvement per unit of time and other improvements were directed to improved quality and yield of benzene polycarboxylic acid product. In general the improved modes of conduct for said liquid phase oxidation using the unique catalysis involved selective order of addition of catalyst components; scheduling addition of catalyst components; use of either constant temperature or constant pressure; scheduling different rates of oxygen supply; use of sources of oxygen having oxygen contents below and above the oxygen content of air; regulation of water content of acetic acid in the oxidation zone; sequentially staging of two or series connected oxidation zones operated at different temperatures, pressures, oxygen concentration or water concentration; usage of different combinations of heavy metals and types of bromide iron source, e.g. ionic and combined bromine; and combinations thereof as applied to batchwise, semi-continuous and continuous operations. Such improved modes of operation using the unique catalysis did provide for increase of benzene di- and tricarboxylic acid products from the yields demonstrated by the methods of U.S. Pat. No. 2,833,816. For example the yields of iso- and terephthalic acids from the corresponding xylenes were increased to 90-92 mole percent from 75-80 mole percent demonstrated by said patent and with attendant improved decrease of partially oxidized xylene such as aldehydo-benzoic acid and toluic acid which contaminated said phthalic acid products.
Commercially feasible methods were devised for removal of color bodies and/or oxygen-containing aromatic compounds contaminating benzene di- and tricarboxylic acid products from such improved modes of conduct of the aforementioned liquid phase oxidation using the unique catalysis. Such purifications were directed either to obtension of light colored or white products intended for use in unsaturated polyesters or to obtension of highly pure terephthalic acid product at least 99.9 weight percent purity for direct reaction with a diol in the manufacture of high molecular weight polyesters required for film and fiber manufacture. However, little attention was given since 1958 to the introduction of a new component into the unique catalytic liquid phase oxidation to decrease the appearance of contaminants which are partial oxidation products attendant the oxidation of xylenes to phthalic acid di- and trimethylbenzenes products recovered from said oxidations.
It has been known since 1958 that oxidation of m- or p-xylenes in the presence of the unique catalysis at temperatures in the range of 50.degree. to 120.degree.C. using oxygen gas as oxidant produced iso- or terephthalic acid products containing relatively large amounts, 3 to 10 weight percent, of 3 or 4 -formylbenzoic acid and like amounts of m- or p-toluic acids. Oxidations conducted under liquid phase conditions in the presence of the unique catalysis at temperatures above 120.degree.C., i.e. in the range of 120.degree. to 275.degree.C., using oxygen gas or air as source of molecular oxygen did decrease the concentration of formylbenzoic and toluic acid contaminants in iso- and terephthalic acid products from the corresponding xylenes to below 3 weight percent, e.g. 0.5-2.0 weight percent. The improved modes of conduct of the catalytic liquid phase oxidation did little to improve the low temperature (i.e. 50.degree.-100.degree.C.) oxidations.
In addition to aldehydic acid and toluic acid contamination of phthalic acids there is also the problem of contamination by catalyst metals and corrosion metals, a problem unique to preparation of halophthalic acids. Such metals are precipitated by the halophthalic acid during oxidation and cause early termination of the oxidation of haloxylenes.
To make more effective, on a pounds per hour throughput basis, the various commercially available purification routes for phthalic acids it is highly desirable to obtain such acids direct from oxidation in a higher quality by some means in addition to such improved modes of conduct of liquid phase oxidation using the unique catalysis.